The present invention relates to a method of and an apparatus for conducting plasma welding. More particularly, the invention is concerned with plasma welding method and apparatus suitable for all-position welding of an object, such as a circumferential joint of a pipe, which requires uniform penetration bead formed by, for example, keyhole welding.
Plasma welding method has been put into practical use in recent years. This welding method features a higher energy density of arc and a greater length of arc so that welding can be effected with higher heat input rate and, hence, at higher efficiency than conventional welding methods such as TIG (Tungsten-Inert-Gas) welding.
Briefly, the plasma welding employs a plasma jet of a high temperature which penetrates a base metal to form a keyhole in the base metal while arc welding is executed. Plasma welding apparatus which have been used hitherto have been designed for flat welding or down hand welding.
Recently, however, there has been an increasing demand for all-position welding. All-position welding by plasma welding method, however, requires a considerably high level of skill because the plasma welding apparatus was intended for a single purpose skill such as keyhole welding or surface fusion welding.
Actual welding operation by plasma welding method is conducted by a licensed operator. The operator is required to handle different types of welders and other machines in different steps of the welding process, such as tack welding, pre-heating, keyhole welding and finish padding.
More specifically, tack welding is conducted for the purpose of fixing welding objects together in advance of the welding and also for the purpose of preventing any thermal distortion which would otherwise be caused by the heat applied during the welding. The tack welding has thus been conducted typically by TIG welding. The pre-heating is necessary for preventing cracking which may occur as a result of contraction of the weld metal during solidification thereof and also for avoiding any influence of welding heat which may cause an undesirable effect on the metallurgical structure of the base metal. The pre-heating has been effected typically by induction heating or by means of flame of a burning gas.
Thus, the known plasma welding method requires various preparatory operations such as tack welding and pre-heating and, in addition, no specific consideration has been given hitherto for the application of the plasma welding method to a welding object which requires a strict control of the welding operation, e.g., all-position welding of pipe joints. In consequence, it has been necessary that a licensed operator conducts the different steps such as the tack welding and the pre-heating by employing specific devices such as a TIG welder and an induction heating apparatus or a gas burner. Alternatively, these different steps have been carried out respectively by making use of different single-purpose robots designed specifically for carrying out such steps.
TIG welding which has been used conventionally provides only a small heat input rate so that the localized portions of members to be welded together are molten uniformly and slowly. For the purpose of successfully carrying out the tack welding, therefore, it is necessary to oscillate the arc between these two members or to insert an additional welding wire so as to increase the amount of the melt of the welding metal. In any case, the welding operation has to be conducted under careful visual check of a skilled person.
The necessity for the preparatory works such as tack welding and pre-heating, which requires a high degree of skill and high strictness of check, seriously impairs the efficiency and causes an impediment on the effort for saving labor when plasma welding is applied to all-position welding of circumferential pipe joints.